The Colonsay Group and basement–cover relationships on the Rhinns of Islay, Inner Hebrides

Synopsis Detailed mapping on the Rhinns of Islay has prompted a revision of the stratigraphy of the lower part of the Colonsay Group (Neoproterozoic). Previously unrecorded metasedimentary rocks exposed along the SE coast rest upon and are intersliced with the Rhinns Complex (Palaeo-proterozoic basement). These metasedimentary rocks, here termed the Octofad Sandstone Formation, are considered to represent the lowest preserved part of the Colonsay Group. On the central-west side of the Rhinns, the Colonsay Group comprises four formations (Eilean Liath Grit, Kilchiaran Phyllite, Rubha Gaidhealach Grit and Rubha na h-Airde Moire Phyllite) with a total stratigraphical thickness of c. 550 m. The stratigraphy is repeated by large-scale NE–SW-trending upright folds. The basement-cover contact in central-west Islay is a major tectonic dislocation, here termed the Kilchiaran Shear Zone. An unknown thickness of stratigraphy has been excised along this contact. There has also been extensive basement–cover interslicing along the SE coast of the Rhinns.